The present invention relates to pond based wastewater treatment systems. In particular, the present invention relates to a wastewater treatment system in which an in-pond clarifier acts as a baffle to separate two distinct treatment zones and in which the in-pond clarifier is provided with separate solid removal means for returning settled solids to a treatment zone and separate solid removal means for wasting settled solids from the clarifier to eliminate mechanical means required for collecting the settled solids.
Wastewater may originate from a variety of sources both domestic and industrial. The characteristics of the particular wastewater will vary depending upon its source. The characteristics of the wastewater are typically quantified by principal treatment parameters including biochemical oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS), nitrogen (N), phosphorus (P) and other pollutants. Treatment of the wastewater reduces the concentration of these parameters to within acceptable ranges.
Treatment of the wastewater may be performed with a variety of different treatment methods or processes. Each method or process produces different by-products and different concentrations of the treatment parameters. Each treatment process may involve several treatment stages whereby the concentration of the measured parameters is reduced to acceptable limits. The treatment stages may be separated from one another by use of a serpentine channel employing a plug flow arrangement or by batch processing.
In some systems, wastewater being treated in one particular process or within one particular treatment stage must be isolated or substantially separated from consecutive treatment processes or stages. For example, treatment stages utilizing aquatic plants such as duckweed which take in nutrients from the wastewater preferably are provided with a quiescent environment. The quiescent environment allows the aquatic plants to form a mat or uniform cover over the wastewater surface and also allows sedimentation of solids and other particulates. In contrast, treatment stages which employ aerobic, anoxic or anaerobic suspended growth processes preferably agitate or mix the wastewater to increase treatment efficiency. Thus, treatment stages utilizing aquatic plants and treatment stages utilizing aerobic, anoxic or anaerobic suspended growth processes are preferably isolated from one another.
Isolation of the wastewater within one treatment process or stage from wastewater within an adjacent to treatment process or stage has typically been achieved by employing separate containment structures or ponds for each different process or stage or by employing a hydraulic baffle to compartmentalize a single containment structure pond into several isolated treatment zones. Using separate containment structures or ponds requires additional land space and increases the cost of such a system.
Employing a hydraulic baffle to separate treatment processes or stages is less expensive and permits existing ponds to be used in the selected system. Hydraulic baffles typically comprise a single wall extending from the wastewater surface to the bottom of the containment structure or pond. In addition to isolating treatment processes or stages, baffles are also used to provide a channelized, serpentine flow to maximized detention time within the system. Baffles are typically made from a variety of materials including concrete, wood or fabric.
Natural wastewater treatment may be achieved by an anaerobic process, an anoxic process, an aerobic process or a combination of the treatment processes. To anaerobically treat wastewater, the wastewater must be exposed to anaerobic micro-organisms or bacteria. The bacteria reproduce and convert organics in the wastewater into the additional by-products of carbon dioxide and methane. This conversion is done in the absence of O.sub.2, NO.sub.3 and SO.sub.4. During this conversion, protons are transferred to carbon atoms to form methane. Phosphorus is conditioned for removal in downstream aerobic processes.
To provide anoxic treatment, the wastewater must be exposed to facultative micro-organisms or bacteria. The bacteria produce and convert organics in the wastewater into additional by-products of carbon dioxide and methane. This conversion is done in the absence of O.sub.2. During this conversion, methane and nitrate are converted to nitrogen gas.
Similar to anaerobic wastewater treatment, aerobic wastewater treatment utilizes bacteria which contact and convert organics within the wastewater to treat the wastewater. However, in contrast to anaerobic wastewater treatment, aerobic wastewater treatment utilizes aerobic microorganisms or bacteria which are active in the presence of free oxygen. The aerobic micro-organisms or bacteria reproduce during the process and convert organic matter into the additional by-products of CO.sub.2, NO.sub.3 and NO.sub.2.
Although anaerobic, anoxic and aerobic wastewater treatment processes utilize different bacteria and result in different by-products, all of the processes rely upon contact of micro-organisms or bacteria with organics within the wastewater. Thus, in all of the processes it is important that contact between the bacteria and the wastewater is maximized by mixing the wastewater and the bacteria.
During anaerobic, anoxic and aerobic digestion of organics within the wastewater to be treated, the bacteria become attached to solid waste, other bacteria or other media suspended within the wastewater. As a result, bacteria may become clustered within distinct areas or pockets of the wastewater. Therefore, the wastewater is typically mixed to maximize the volume of wastewater coming into contact and being digested and converted by the bacteria.
The time the process requires to convert organics and the system's efficiency for removing organics depends upon the number of bacteria in the wastewater during treatment. Moreover, because bacteria die, the bacteria count is also critical to the survival of the system. Thus, any anaerobic, anoxic or aerobic process must ensure that the number of bacteria in the system is maintained. However, removing the effluent from the system also removes many of the bacteria suspended in the wastewater. This loss of bacteria, if left uncorrected is known as insufficient solids retention and results in poor treatment efficiency and eventual failure of the anaerobic, anoxic or aerobic wastewater treatment system.
In an attempt to prevent insufficient solids retention, conventional systems have employed reactors with large detention times, reactors with fixed media or reactors with intensive management of microbial populations. Other reactors or digestors have attempted to increase solids retention by the addition of a settling step in the process whereby solids containing the bacteria (activated sludge) are settled out of the wastewater and are returned to the system (return activated sludge). Conventional systems employ external settling in a clarifier to achieve solids retention. However, these systems are not efficient because transportation of the wastewater to an external settling clarifier alters the settling characteristics and increases the amounts of solids suspended in the wastewater. Consequently, more time is needed to sufficiently settle out the bacterial solids.
Another system which uses settling is a sequencing batch reactor. These reactors use internal settling to achieve a high efficiency or organic removal. However, the batch reactor is incapable of handling a continuous flow of wastewater. Instead, the wastewater must be treated in batches. As a result, a wastewater storage facility is normally required for holding effluent while a batch reacts.
Other systems have attempted to achieve increased solids retention through the use of filters and sludge blankets. Systems using anaerobic filters require media to increase solids retention. These systems have a high efficiency of removal and are resilient to shutdowns and temperature changes. However, the required media is expensive. Systems employing an up-flow anaerobic sludge blanket reactor use a floating blanket of sludge to remove bacterial solids from the effluent. Although this achieves a high efficiency of removal, such a system is difficult to manage.
Although it is important to prevent excessive loss of aerobic bacteria from an aerobic wastewater treatment system, it is also important that a number of aerobic bacteria within the system does not become excessive. An excessive number of aerobic bacteria in the system produces an unhealthy biological culture and thereby reduces the system's effectiveness at treating the waster water. As a result, solids or activated sludge containing the aerobic bacteria must be periodically removed or wasted from the system (waste activated sludge). Typically, one to five percent of the activated sludge or settled solids are wasted from the system. The waste activated sludge is typically transferred to a biosolids retention or treatment facility. Treatment of the waste activated sludge requires that the waste activated sludge be highly concentrated and thickened. As a result, activated sludge is typically thickened within the clarifier before being removed from the system.
Examples of clarifiers which return activated sludge to the system and which also waste activated sludge from the system include floating clarifiers and integral clarifiers. Conventional floating clarifiers employ elaborate thickening and collection mechanisms for removing solids from the in-pond clarifier. Floating clarifiers are conventionally positioned along the perimeter or side of the pond opposite the inlet and adjacent to the outlet so that the elaborate thickening and collection mechanisms can be accessible from the perimeter of the pond. A typical floating clarifier comprises a metallic enclosure which defines a clarifier compartment having a V-shaped cross-section. A rake is used to collect and thicken all of the settled solids along the compartment bottom so that at least a portion of the settled solids may be wasted. A single air lift pump inlet device extends along the entire bottom of the clarifier compartment. The single air lift pump removes both return activated sludge and waste activated sludge from the bottom of the clarifier compartment. Once removed from the clarifier, the settled solids or sludge is divided into two portions. A first portion (return activated sludge) is returned to the treatment pond while a second portion (waste activated sludge) is wasted from the system. Typically, the return activated sludge is returned to the pond through a single outlet.
Integral clarifiers are typically formed as part of the containment structure or pond and are formed along the perimeter or boundary of the containment structure adjacent to the effluent outlet. Integral clarifiers typically comprise a partition wall which floats adjacent to the wastewater surface and extends downward to separate the integral clarifier from the rest of the pond. Influent to the integral clarifier flows between the floor of the pond and the partition wall. A hopper is formed at the bottom of the integral clarifier to simplify sludge concentration and removal. Similar to floating clarifiers, integral clarifiers include a single or dual air lift pump which extends along the hopper bottom and a flocculation rake. The flocculation rake is used to thicken all of the settled solids along the hopper bottom so that at least a portion of the settled solids may be wasted. The single or dual air lift pump removes both return activated sludge and waste activated sludge.
Because conventional floating clarifiers and integral clarifiers utilize a single or dual air lift pump to remove both return activated sludge and waste activated sludge, which must later be separated, and because conventional floating clarifiers and integral clarifiers are located on a perimeter or edge of the treatment pond or containment structure, several inefficiencies result. Because both clarifiers utilize a single or dual air lift pump to remove both return activated sludge and waste activated sludge together, equipment must be provided for splitting or separating the return activated sludge and the waste activated sludge within the single outflow stream of sludge. Furthermore, because the return activated sludge and the waste activated sludge are not separated until after they are removed from the clarifier, both portions of the settled solids must be highly concentrated and thickened before being removed from the clarifier. Although waste activated sludge typically comprises between about one to five percent of the total settled solids, all of the settled solids must be thickened and concentrated. Thickening and concentrating the return activated sludge is counterproductive since the concentration of the return activated sludge is preferably low to enhance mixing of the return activated sludge with the wastewater once it is returned to the wastewater. Because the return activated sludge is returned to the pond through a single outlet, the solids returned to the treatment pond are further concentrated and require additional mixing to sufficiently disperse the bacteria within the wastewater.
At the same time, because it is also necessary that all of the settled solids be thickened and concentrated as much as possible, both conventional clarifiers include a rake which mechanically operates adjacent to the floor of the clarifier compartment to thicken and concentrate all the settled solids adjacent to the floor of the compartment. Because the rake constitutes a mechanical component which must be moved adjacent to the floor of the clarifier compartment, both clarifiers are expensive and difficult to maintain. Moreover, because the rake typically contacts and engages the walls of the clarifier enclosure or hopper, the walls of the clarifier enclosure or hopper must be made of a rigid, stronger material such as metal or concrete to prevent tearing of the enclosure or hopper walls. As a result, the enclosure or hopper is also more expensive, heavier and difficult to transport and set up.
Furthermore, because floating clarifiers and integral clarifiers are typically positioned adjacent to a perimeter of the pond, solids cannot be easily returned to an intermediate location within the pond to further enhance remixing of the return activated sludge with the wastewater. Instead, conventional clarifiers require extensive return activated sludge piping. Consequently, a large amount of energy is required for returning the return activated sludge to the pond. This additional energy is typically supplied by a subsidiary pump, such as a centrifugal pump, outside of the clarifier. The additional pumping required to return the solids shears the interconnections between the bacterial solids (also known as floe) to reduce particle size. As a result, future settling of the bacteria is more difficult because of the smaller floe or particle size.